Method of controlling continuous casting



United States Patent Inventors Armin Thalmann Qster and Richard Moser,Dietikon, Switzerland 816,850

Dec. 23, 1968 Division of Ser. No. 606,147, Dec. 30, 1966, abandoned mix1970 Concast A G,

Zurich, Switzerland Dec. 30, 1965 Switzerland Appl. No. Filed PatentedAssignee Priority METHOD OF CONTROLLING CONTINUOUS CASTING 8 Claims, 20Drawing Figs.

US. Cl 164/4, 164/155 Int. Cl B22d 11/10, B22d l/02, B22d 17/32 Field ofSearch 164/4, 82,

[56] References Cited UNITED STATES PATENTS 2,145,438 H1939 Thulin164/152 3,300,820 l/1967 Tiskus et al 164/155 3,344,841 10/1967 Rys etal 164/154X 3,349,834 10/1967 Wilson 164/155 3,358,743 12/1967 Adams164/154 FOREIGN PATENTS 1,373,146 4/1964 France 164/154 PrimaryExaminer-J. Spencer Overholser Assistant Examiner- R. Spencer AnnearAttorney-Sandoe, Neill, Schottler & Wikstrom ABSTRACT: In a continuouscasting plant sensing elements a responsive to the metal supply to themold, to the level of metal in the mold or in the partly solidifiedcasting, and/or to a breakthrough of liquid metal from the castinggenerate signals that are automatically analyzed and applied to controlthe supply of metal to the mold, the rate of withdrawal of the cast-ving from the mold and/or the cutting of the casting.

Patented Nov. 3, 1970 Sheet 1 ATTORNEYS Patented Nov. 3, 1970 3,537,505

Sheet 3 01 6 49 '49 FIGZ) 52 50 I 5/ 68 66 69 29 l 60 30 RICHARD MOSER ulkamcwa wwfl ATTORNEYS Patented Nov. 3, 1970 Shoot F|G.H

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ATTORNEYS Patented Nov. 3, 1970 I 3,537,505

Shoot 6 Of 6 INVENTORS ARMIN THALMANN RICHARD MOSER ATTORNEYS METHOD OFCONTROLLING CONTINUOUS CASTING This is a division of application Ser.No. 606,147 filed Dec. 30,1966, now abandoned.

A signal indicating a fall in the level of molten metal in acontinuously cast strand or in the molten metal in the continuouscasting mold is fed to a switching network which analyzes the type offault and automatically initiates control signal for plant correctingcontrol.

The present invention relates to a method of controlling a continuouscasting plant when a fault in operation arises which results in a fallin the level of the metal pool in the mold.

Continuous casting plant are usually operated by manual control means.Automatic controls that have been proposed include arrangements forweighing the metal for the purpose of checking the level of the metal inpouring vessels, as well as arrangements for monitoring the level of themetal pool in the mold. The first of these devices is used forcontrolling the rate of metal supply and the second for controlling therate of withdrawal of the casting from the mold.

Some of the work of manual control is undertaken by electrical elements.In order further to reduce the work involved in an manual control infavour of automatic control, means must be provided for monitoring andindicating operational faults which may occur whilst casting proceeds.

The most troublesome fault during continuous casting is an eruption orbreakthrough of liquid metal which may be due to unpredictable causes ofa metallurgical and mechanical kind.

Moreover, during continuous casting troubles may arise in connectionwith the supply of liquid metal to the casting mold. One type of fault,caused for instance by a gating valve jamming in open position, may leadto an excessive supply of liquid metal into the mold. It has alreadybeen proposed to provide monitoring means which generate a signal whenthe level of the liquid pool in the mold rises beyond a fixed maximumlevel and to use the signal for activating an alarm.

In a different group of faults, due for instance to a partial blockageof the ingate by a fragment off the plug, the supply of metal to themold is insufficient.

A known arrangement for monitoring the level of the liquid pool in themold seeks to indicate such a deficiency by generating a signal when thelevel of the pool falls below a minimum level, the signal beingsimilarly used to activate an alarm or to reduce the rate of withdrawal.

However, it has not yet been recognised that a fall in the level of theliquid pool may also be due to other causes which call for immediateremedial action. A fall in the level of the liquid metal also occurswhen there is a breakthrough of metal through the crust of the partlysolidified casting. Moreover. when casting begins and when casting endsliquid metal is ubsent at the minimum level. Consequently the signalderived from such a minimum level monitoring device has no uniquemeaning, in other words it does not indicate the nature of the fault orthe operating conditions causing the indicated condition.

It is therefore the object of the present invention to provide controlmeans for a continuous casting plant which respond to a breakthrough ofmetal as well as to a deficiency in the rate of metal supply in suchmanner that the appearance of a corresponding signal will permitimmediate corrective controlling action to be initiated.

According to the invention this is achieved in that at least one elementis provided which generates a signal characteristic of the type of faultthat has occurred and that by reference to said signal the plant iscontrolled in the manner required for dealing with the particular fault.

The system for performing this method is characterised by electricalswitching elements which evaluate the signal of the element indicatingthe fault, and which are associated with storage elements.

For unambiguously indicating the occurrence of breakthrough of metal thesignal reporting the fault may be derived from an element which respondsto it fail in the level oithe liquid metal in the partly solidifiedcasting. Alternatively this signal may also be drived from an elementwhich responds to the liquid metal issuing from the partly solidifiedcasting.

For unambiguously indicating a deficiency in metal supply to the moldthe fault signal may be derived from an element which responds to a fallin the level of the metal pool in the mold. Alternatively this signalmay be derived from an element which monitors the size of the teemingmetal jet.

For generating a signal indicating a fall in the level of the metal inthe partly solidified casting or in the level of metal pool in the moldthree possibilities are available:

a. A level indicator may be arranged to supply an analogous signal. Therate at which the level of the metal falls may then decide which kind offault signal must be generated.

b. A level monitoring means may be arranged to supply signals at twolevels. The signal magnitudes at these levels may then be used to decidethe nature of the fault generating the signal.

c. Digital signals may be generated at two levels. The nature of thefault signal is then determined by the interval in time between thesignals at the two levels in relation to a predetermined time interval.

The signals generated by the proposed elements may also be subject tothe effects of other signals, as will be later described, for bridgingthe operating states which arise at the beginning and end of the castingprocess and for avoiding unwanted fault signuls during these phases.

Other features of the invention will be understood from the followingdescription of embodiments of the invention with reference to thedrawings in which:

FIG. 1 is an elevation cross sectioned view of a continuous castingplant, with sensing and control circuitry in schematic form;

FIG. 2 is a cross section of a continuous casting mold with monitoringmeans providing two level indications, and comprising a radiation sourceand radiation detectors;

FIG. 3 is a cross section of a continuous casting mold with monitoringmeans providing two level indications, and comprising a transmitting anddetecting system for electromagnetic waves;

FIG. 4 is a cross sectioned view of a cast strand having two elementsresponsive to flowing metal from a breakthrough;

FIG. 5 is a side elevation of the system according to FIG. 4;

FIG. 6 is a cross section view of a continuous casting plant with apyrometer responsive to the size of the teeming jet;

FIG. 7 is a cross sectioned view of a continuous casting plant with amagnetizing coil responsive to the size of the teeming jet; 7

FIG. 8 is a schematic diagram of the electrical system for evaluatingthe signal from one level according to FIG. 2;

FIG. 9 is a plot of the wave forms at different points of the circuit ofFIG. 8 illustrating the form of signal obtained when a breakthroughoccurs;

FIG. 10 is a schematic diagram of the electrical system for evaluatingthe signals derived from two levels according to FIG. 2;

FIG. 11 is a plot of wave forms at different points in the circuit ofFIG. 10 illustrating the form of the signals obtained in the event of abreakthrough;

FIG. 12 is a plot of wave forms at different points in the circuit ofFIG. 10 illustrating the form of the signals obtained in the event of adeficient metal supply;

FIG. 13 is a schematic diagram of an electrical system for evaluatingsignals received from elements according to FIG. 3;

FIG. 14 is a plot of wave forms at different points of the circuit ofFIG. 13 illustrating the form of the signals in the event of abreakthrough;

FIG. 15 is a plot of wave forms at different points of circuit of FIG.[3 illustrating the form of the signals in the event of a deficientmetal supply;

FIG. I6 is u schematic diagram of an electrical system for evaluatingthe signal from temperature-responsive elements according to FIGS. 4 and5;

FIG. 17 is a schematic diagram of an electrical system for evaluatingsignals from fusible members according to FIGS. 4 and 5;

H0. 18 is a schematic diagram of an electrical system for evaluatingsignals obtained from a pyrometer according to FIG. 6;

FIG. 19 is a schematic diagram of an electrical system for evaluatingsignals obtained from a magnetizing coil according to FIG. 7; and

FIG. 20 is a schematic diagram of an electrical circuit layout forcontrolling the drive means and the servo elements which regulate thesupply of steel.

In FIG. 1 there is shown a continuous casting plan plant with a curvedmold and a curved casting guide, comprising a pouring ladle 1 with aplug 2 controlled by servo means 3. The metal flows from the ladle intoa further intermediate pouring vessel or tundish 4. The gate of thetundish 4 is controlled by a slide valve 5 operable by servo means 6.From the tundish 4 the metal flows into a curved water-cooled continuouscasting mold 7 in which a continuous casting 8 with a liquid core forms.This casting -8 is withdrawn from the mold 7 by withdrawing rollers 12and is assisted by rollers 10 through the casting guide in which thecasting is further cooled by sprayers 11. After having passed betweenthe withdrawing rollers 12 which are driven by drive means 13 thecasting 8 is cut into lengths by a cutting device. The cutting device 14is associated with synchronising gear 15 which moves the cutting deviceat the same speed as a casting during the cutting operation. Thesynchronising gear 15 is driven by drive means 16. The cutting device 14is also equipped with traversing gear 17 which guides and moves thecutter burner across the axis of the cast- The servo means 3, forexample in the form of a motordriven lifting gear for raising andlowering the plug 2, is controlled by a controller 25. The tundish 4 ispreferably fitted with weighing equipment in the form of load gauges 26.The load gauges 26 permit the level of the metal surface in the tundish4 to be inferred from the weight of the metal. Preferably the loadgauges are associated with means which respond to three levels of themetal surface, namely a maximum level, a minimum level and an absoluteminimum level. Response to these levels affects the controller andresults in actuation ofthe plug 2. At maximum level the plug 2 isclosed, at minimum level it is opened. The effect of the absoluteminimum level will be later described.

The servo means 6, which may be a hydraulic actuator for operating theslide valve 5 is controlled by a controller 27. The top of the mold 7 isfitted with level monitoring means 28, preferably comprising aradioactive source in the form of a rod and radiation detectors. Thelevel monitoring means 28 respond to two levels ofthe metal pool in themold 7, namely a maximum level and a minimum level. These two levelindications are applied as signal 030 to an input of the controller 27and cause the slide valve 5 to be operatedpAt maximum level the valve isclosed and at minimum level it is opened. However, level monitoringmeans comprising rod sources are also suitable for continuously checkingthe volume of metal.

The drive means 13 for the withdrawing rollers 12 are controlled by acontroller 31. This controller 31 which determines the normal rate ofwithdrawal of the casting can be influenced by information 032 whichneed not here be specified in detail, such as information relating tothe thickness of the crust, the temperature ofthe casting and so forth.

At 33 is a further controller which controls the two drive means 16 and18 associated with the cutter device 14. This controller 33 whichdetermines the normal operation of the cutter operates by reference toinformation 034 which need likewise not here be described in detail,such as the length of the portions to be cut off, the temperature of thecasting and so forth.

For the generation of signals indicating the presence of faults FIG. 1illustrates three arrangements feasible for such applications:

d. For indicating a breakthrough of metal temperature responsiveelements 41 such as thermoelements may be provided between the twocooling members marked 9; Should any metal break through the crust ofthe partly solidified casting these elements generate a signal 041.

In additionally or alternatively fusible members 42, such as metalbandstensioned by springs may be provided at this point. if liquid metalbreaks through, these likewise generate a signal 042.

e. Near the bottom end of the mold 7 are two level indicators 43 and 46comprising a radioactive source in rod form and a radiation detectorassociated with each level. When the level of the metal in the partlysolidified casting or the level of the metal pool falls below the levelsof these indicators 43 and 46 signals 045 and 048 are generated.

Further level indicators in the form of a transmitting and detectingsystem 49 for electromagnetic waves are provided above the mold 7. lfthe level of the liquid metal in the mold falls to critical levels atwhich the rays are reflected at the metal surface so that they fall intoone of the detectors, the latter will generate a signal 051 or 052respectively.

f. A deficiency of metal supplied to the mold 7 may be detected bymonitoring the size of the teeming metal jet. A deficient supply may bedue to one of several reasons. For instance, the gate may freeze, theflow of metal through the gate may be obstructed by a fragment of theplug, a slide valve may freeze and so forth. For monitoring the size ofthe teeming jet a pyrometer 53 is provided above the mold 7, whichresponds to the light radiated by the jet. This pyrometer generates asignal 053.

Alternatively, as indicated in H05. 7 and 19, atthe same point above themold a magnetizing coil 54- may be provided if the gate is provided witha spout extending to the surface of the pool. A signal 054 can then begenerated which reflects the magnitude of the eddy current loss in theliquid metal.

The manner in which use can be made of these three possibilities inprinciple of generating signals will be later described in greaterdetail with reference to Us. 2 to 7. According to the arrangementsprovided, the signals 041, 042, 045, 048, 051, 052, 053, and 054 (FIG.T9) are fed to a safety controller 55.

Other information may also be fed to this controller at the same time.lnformation of such a kind may comprise:

g. A signal 026 indicating the absolute minimum level of metal in thefeed head; i

h. A signal 020 indicating that casting is in progress in the plant; and

i. A signal 030 derived from the minimum level indicator of the poolmonitoring means.

The signals enumerated in points g, h and i will be referred to later.

The controllers 25, 27, 31 and 33 operate under the control of thesafety controller 55 which in the event of a fault also activates analarm 56 when there is a breakthrough of metal or an alarm 57 wherethere is a deficiency of metal supply.

FlG. 2 illustrates the provision, at the lower end of the mold 7, of twolevel indicators 43 and 46, comprising a rod-shaped source 44 and tworadiation detectors 45 and 48. Each detector 45, 48 may consist of acadmium sulphide crystal which varies its electrical resistanceaccording to the radiation dose which it receives. In normal operationthe level of the pool of metal in the partly solidified casting 8 shouldbe as indicated by the dashed line 60. Since the radiation is absorbedby the metal at the detector levels 43 and 46 the resistances of thedetectors will be high. However, if, as a consequence of a breakout ofliquid metal, the level of the metal in the partly solidified casting 8falls to the level 61 below the two detector levels 43 and 46, theabsence of liquid metal in the partly solidified casting 8 in the pathof the rays results in the detectors being exposed to a higher dose ofradiation with a resultant drop in resistance. The detectors 45 and 48thus generate signals 045 and 048 which are processed to form abreakthrough signal in a manner that will be later described.

The same arrangement may also be used for the generation of a signalwhen there is adeficiency in the rate of metal supply. in such a casethe surface of the pool will fall below the detector level 43 or 46without leaving a solidified marginal zone. Consequently the resistancesof the crystals in the detectors 45 and 48 will in such an event bestill lower. Alternative ly the detector 28 which monitors the level ofthe pool may take the place of one of the two detectors 45 or 48.

The arrangement according to FlG. 3 consists of a trans mitting anddetecting arrangement 49 for electromagnetic waves, for example, forradar, laser or other beams. in normal operation the level of the poolin the mold 7 will be as indicated by the dashed line at 60. Theelectromagnetic beam 66 projected by a transmitter 50 in the directiontowards the surface of the pool is reflected at the surface level 60,and the reflected beam will be situated as indicated by the dashed line67. This reflected beam does not affect either of two detectors 51 and52. However, if as a result of a deficiency in metal supply the level 60of the surface of the pool falls to the socalled first critical levelindicated by the dashed line 62, then the beam 66 will be reflected atthe surface of the pool in the direction marked 68 and the reflectedbeam will now fall on the detector 51 with a given angle of incidence. Asignal 051 will therefore be generated.

if the surface of the pool continues to fall until it reaches the secondcritical level 63 then the beam 66 will be reflected at the surface ofthe pool in the direction 69 and fall on the second detector 52 with agiven angle of incidence. When this occurs a signal 052 will thereforebe generated.

The same elements will generate the same signals 051 and 052 if thelevel of the metal in the partly solidified casting falls as the resultof a breakthrough of metal. As will be later described the time intervalbetween these signals decides whether they will be processed to form asignal for indicating a breakthrough or a deficiency in metal supply.

Instead of providing transmitting and detecting means forelectromagnetic waves, ultrasonic devices could be used in an analogousway.

For the sake of completeness FIG. 3 also shows monitoring means 28 forthe level of the pool in the form of a rod source 29 and a detector 30.

N05. 4 and 5 illustrate two methods of generating signals when liquidmetal breaks through the crust of the partly solidified casting 8. a themold is a secondary cooling zone comprising cooling members 9. In theregions where breakout of liquid metal could occur, i.e. in the zones 75and 76, tempcrature responsive elements 41 and/or fusible members 42 maybe provided. Since the temperature responsive elements 41 can monitoronly part ofthe circumference of the casting 8 several such elements aredistributed around the casting periphery.

in the event of a breakout of liquid metal some of the tem peratureresponsive elements 41 will be contacted by the flowing liquid metal andthey will then generate a signal 041 which will be further discussedwith reference to HQ. 16.

Similarly, any flowing metal will contact some part of the fusiblemember 42 and cause the generation of a signal 042 which will be furtherdiscussed with reference to FIG. 17. Preferably the fusible members 42are kept under tension by springs 77 to ensure that the electricalcircuit will be actually ruptured when the fusible members melt. Formonitoring the entire periphery of the casting in the zone 76 thefusible members may have the form of loops.

FIG. 6 illustrates the method of detecting a deficiency in the rate ofmetal supply by monitoring the size of the jet 80. The emission of lightby the metal which corresponds to the size of the jet 80 affects theresistance of the cadmium sulphide crystui in u pyrometer 53. A networkassociated with this pyromctcr transforms this resistance into aproportional voltage. The voltage obtuinedia therefore a measure of theap proximate size of the teeming jet 80 and consequently represents therate at which metal enters the mold. This voltage is the previouslymentioned signal 053 which indicates a deficiency in metal supply.

FIG. 7 illustrates an arrangement for monitoring the size of the teemingjet with the aid of a magnetizing coil 54. The coil surrounds a gate 82which extends from the tundish 4 to the level 60 of the pool. Analternating current which flows through the coil 54 varies in magnitudein proportion to the cross section of the metal flowing through the gatein accordance with the eddy current loss experienced in the metal. Thecurrent is transformed into a proportional voltage in a discriminator.These voltages may similarly provide the signal 054 which represents adeficiency in metal supply.

FlG. 8 illustrates the circuit in which the signal 045 is processed toform a signal 093 indicating a breakthrough of metal. A current drivenby a voltage source 85 in the detector 45 flows through the cadmiumsulphide crystal 86 and a resistor 87. The voltage drop across theresistor 87, i.e. the signal 045, is fed to a differentiating member 88which delivers a signal 088. This signal is applied to a Schmittcomparator circuit 8 if the signal 088 which has been differentiatedwith respect to time, rises above a given threshhold 90 (FIG. 9), i.e.if the rate at which the liquid metal surface falls exceeds thethreshold 90 which is determined by the rate of withdrawal, then signal089 will appear in the output of the comparator circuit. lnorder to makeallowance for variations in the rate of withdrawal the theshhold value90 is preferably arranged to change in accordancewithany suchvariations. The signal 089 is taken to an amplifier 91. The amplifieroutput is connected to a storageelement in the form of a self-holdingrelay 92. The relay is therefore capable of responding to and ofretaining the signal 089 which normally appears in the form of a pulse.The

signal 093 for indicating the occurrence of a metal breakthrough isgenerated by the closure of a contact 93 operated by the relay 92. Acancellation device not shown in the drawing permits this signal 093 tobe extinguished.

FIG. 9 is a diagram illustrating the signals as a function of time thatare generated by a breakthrough of liquid metal in solid line; thesignals due to a deficiency in supply are also plotted in dashed linesfor a comparison.

As can be seen from the plot of signals, for clearly indicating adeficiency in metal supply the illustrated arrangement is unsuitablebecause the rate of change of the signal 045 is then likely to be slow.Under normal casting conditions the rate of change would be nil. Adiscrimination between the two operating conditions is thereforedifficult. In order to overcome this difficulty use is made of a levelindicator employing a temperature responsive detector, whichincidentally is analogously unable to discriminate between abreakthrough and normal casting conditions because of the presence ofthe solidified crust on the mold walls. However, clearly differentiablesignals indicating a deficient metal supply can be obtained because therate of change of the signal when the level of the metal pool sinks isstill fast enough. The signal generated by the temperature responsivedetector is processed in substantially the same way as that describedwith reference to FIGS; 8 and 9.

The circuit illustrated in FlG. 10 comprises two detectors 45 and 48.The detector 48 is in effect exactly equal to the detector 45 which hasbeen described with reference to FIG. 8. 94 is a source of potential, 95is a cadmium sulphide crystal and 96 is a resistor. The signals 045 and048 are each taken to a Schmitt comparator circuit 97 and 99respectively. A signal 097 or 099 will therefore appear in the output ofone of the comparator circuits 97 and 99 whenever one of the signals 045or 048 exceeds a predetermined threshhold value 105 (FIG. 11). Thisthreshhold value 105 roughly corresponds to the magnitude of the signalwhich would be generated by the detector in the presence of a crust andin the absence of liquid metal. A signal 098 or 0100 will appear in theoutput of the corresponding comparator circuit 98 or when the signal 045or 048 exceeds a higher threshhold value 106 (FIG. 12). This latterthreshhold 106 roughly corresponds to the mag nitude of the signal whichthe detector would generate in the absence of both a crust and liquidmetal. From the magnitudes of the signals generated at the level of thetwo detectors by reference to the two threshholds it is possible clearlyto differentiate between a breakthrough and a deficiency in metalsupply.

The two detectors 45 and 48 must be suitably spaced to ensure thatintermediate conditions will not lead to faulty indications.

In the event of a metal breakthrough the signals appearing in theoutputs of the two comparator circuits 9'7 and 99 associated with thelower threshholds consecutively affect an AND-gate 107 which delivers asignal 0107. This signal is further processed to provide a signal 093indicating the occurrence of a breakthrough as has been described withreference to FIG. 8.

If the signals result from a deficiency in metal supply the signalsappearing in the outputs of the comparator circuits 98 and 100associated with the higher threshholds will consecutively affect asecond AND-gate 108 which delivers a signal 0108. In order to avoid thegeneration of an unwanted deficiency signal when casting begins andends, a signal 026 representing the absolute minimum level of the metalin the tundish 4 and a blocking signal 0114 are likewise fed to furtherinputs of the gate 108.

For generating the signal 0114 a storage element 112 is provided whichcomprises NEITHER-NOR gates 113 and 114. The output ofgatc 113 is takento the first input of gate 114, whereas the output of gate 114 is taken,on the one hand, as already mentioned, to one input of the gate 103 and,on the other hand, to the first input of gate 113. The second input ofthe gate 114 receives a signal from a NOT gate 115 of which the inputreceives the signal 020 indicating that casting is in progress. Thissignal may either be triggered by the operator or it may be given byautomatic means, and it continues so long as casting proceeds. Thesignal 030 from the minimum level of the pool level monitoring means isfed to the second input of the gate 113. The signal 020 thus serves totrigger the blocking signal 0114, whereas signal 030 operates toextinguish the blocking signal.

The signal 0108 is further processed to provide the metal deficiencysignal 0111 in the same way as described with reference to FIG. 8,excepting that in the drawing elements 109, 110 and 111 have'beensubstituted.

The FIGS. 11 and 12 are graphs of the locally generated signals whichappear in the event ofa breakthrough or a metal deficiency.

FIG. 13 illustrates the generation of fault signals 093, 0111 from twodifferent levels at which closely consecutive signals (time intervnl inthe order of seconds) 051 and 052 are generated. The signal 051 isapplied, on the one hand, to the input of a time delay element 120 and,on the other hand, to the input of a pulse generating element 121. Thesignal 052 is applied to a further pulse generating element 122. Thesignal 051 therefore appears in the form of a signal 0120 in the outputof element 120 after a period of delay 127 (FIG. 15) and, on the otherhand, as a signal 0121 in the form of a pulse of duration 125 (FIG. 14)in the output of element 121. Furthermore, the signal 052 appears in theoutput of element 122 as a signal 0122 in the form ofa pulse of duration126.

The time 127 comprises the two periods 125 and 126. The period 126represent the response time of the relays 92 and 110.

The period 125 is decisive for discriminating between the type of thefault inasmuch as at a given spacing of the two monitored levels alimiting value for the rate of fall of the metal surface is introduced.This limiting value may with advantage be derived from the rate ofwithdrawal of the casting because it varies with the casting parameters:quality of the steel, format, casting temperature and so forth. If thesignals from the two monitored levels appear consecutively with anintervening interval of say 128 within the period 125, then this meansthat the metal level is falling at a rapid rate in relation to thelimiting value, Le. the rate ofwithdrawal, and that there has been abreakthrough of metal. On the other hand, ifthe interval between the twosignals is say 129 and exceeds the period 125, then this means that thefall of the metal surface is slow in relation to the limiting value,i.e. that the rate of supply of metal is insufficient.

The two signals 0121 and 052 are both applied to the input of anAND-gate 124 which delivers a signal 0124. This signal is furtherprocessed to provide the breakthrough signal 093 in the same way asalready described with reference to FIG. 10.

The two signals 0120 and 0122 as well as the signal 026 indicating theabsolute minimum level of the metal in the tundish 4 are applied to thethree inputs of an AN D-gate 123 which delivers a signal 0123. Thissignal 0123 is again processed to provide a metal deficiency signal 0111in the same way as described with reference to FIG. 10.

The two FIGS. 14 and 15 are graphs illustrating the sequence in time ofthe signals.

FIG. 16 illustrates the electrical circuit used for the generation of abreakthrough signal 093 from the signal obtained from temperatureresponsive elements 41. As soon as one or more of the thermoelements 41are contacted by liquid metal, a low voltage appears at the input of anamplifier 130. After amplification this voltage is applied to a Schmittcomparator circuit 131. A Schmitt comparator circuit has the property ofbeing able to convert a continuously varying voltage, such as thatgenerated by thermoelemcnts, into two different states 0 or I byreference to a given coincidence level. A signal 0131 is thus formed inthe output of the comparator circuit 131.

This signal 0131 is further processed to provide the breakthrough signal093 in the manner described with reference to FIG. 10.

FIG. 17 illustrates the generation of a breakthrough signal 093 from theresponse of fusible members 42. A current is driven by voltage source132 through a resistor 133 and the fusible members 42. The input of acontact protection circuit 134 is electrically short-circuited by thefusible members 42 which are connected in parallel. In the event of abreakthrough, as described with reference to FIGS. 4 and 5, theelectrical path through at least one of the fusible members will be atleast temporarily broken by the liquid metal. A voltage pulse willtherefore be applied to the input of circuit 134 and a signal 0134 willappear in the output. This signal is further processed to provide abreakthrough signal as described with reference to FIG. 10.

FIG. 18 illustrates the manner in which a signal 053 from the pyrometer53 is processed to provide a deficiency signal 0111 in conjunction withthe signal 030 obtained from the minimum level of the pool levelmonitoring device. The signal 053 is first applied to a Schmittcomparator circuit 141. The adjustable reference value of the comparatorcircuit 141 corresponds to a minimum permissible size of teeming jetwhen the slide valve 5 is wide open. A signal 0141 appears in the outputof the comparator circuit. Moreover, the signal 030 is taken to a timedelay element which delivers a signal 0140. During the time determinedby the element 140 the valve has the opportunity of opening fully. Thesignals 0140, 0141, 026 and 0114 are all applied to the inputs ofAND-gate 142 which delivers a signal 0142. This signal 0142 is furtherprocessed to provide a deficiency signal 0111 in the same way asdescribed with reference to FIG. 10.

FIG. 19 illustrates the evaluating circuit for a signal representing thesize of the teeming jet obtained by means of the magnetizing coil 54. AnAC voltage is fed to the coil 54 from a source of AC 145 through aresistor 14.6. The voltage drop across the resistor 146 is convertedinto a DC voltage 0148 in a rectifier bridge 147 and a smoothingcapacitor 148. This DC signal 0148 is applied to an amplifier 149 whichdelivers the signal 054. This latter signal 054 is fed into the circuitshown in FIG. 18 in place of the signal 053 in order to generate thedeficiency signal 0111.

FIG. 20 illustrates the circuitry in the main safety controller 55 whichcontrols the drive means and the servos regulating the supply of steel.A suitable source of voltage is connected to the terminals and 156. Thediagram represents the state of readiness for operation, i.e. the serves3 and 6 are energised in position "closed" and the drive means 13, 16and 18 are without current. Moreover, control elements 160 to 164 formanual control have not been operated.

During the normal casting process the servo 3 or 6 can be operated bycontroller 25 or 27 respectively through a contact 165 or 166respectively for setting them into the positions "open" or closed"according to the level of the metal. Drive member 13 has been started bycontroller 31 via a contact 167. As soon as the desired length ofcasting has passed under the cutter unit 14 the drive means 16 and 18respectively are started in the direction feed" via a contact 168 and169 respectively operated by controller 33. As soon as the casting hasbeen cut the drive means 16 and 18 respectively are changed to return?by a limit switch not shown in the drawings. As soon as the return hasbeen completed another limit switch controls the drive means 16 and 18respectively to stop".

Should there be a breakthrough the relay 92 operates, as alreadydescribed, and closes its contact 93. This causes the breakthroughsignal 093 to be delivered, which starts the alarm 56. At the same timea relay 170 is energised which operates contacts Hi to l74. Thesedisconnect the controllcrs 25, 27. 31 and 33 and render themineffective. Hence the servos 3 and 6 move into position closed" and thedrive means 13 is stoppedlf the cutter is in operation when thebreakthrough signal appears, then the drive means 16 is likewiseinactivated, whereas the drive means 18 is not affected by thebreakthrough signal so that the cutting operation can be completed.

The changeover of the contacts 171 to 174 simultaneously brings thecontrol elements 160 to 164 for manual operation into circuit. The plantis therefore now set up for manual control and the operator canseiectably activate the servos 3 and 6 and the drive means 13, 16, 18 byoperating the contacts of the control elements 160, 161, 162, 163.

if in the cutting region the casting is not supported by rollers whichtravel together with the moving casting a cutting operation that hasalready begun must not be completed if the flame of the cutter wouldthen damage a stationary roller. in order to cope with this situation aroller contact 177 is provided which responds for instance when theroller is excessively heated.

As already described, the relay '110 operates when a deficiency of metalsupply occurs and closes a contact 111. The deficiency signal 0111 isthus generated and indicated by the alarm 57. At the same time a relay175 which operates with delay is energised. After a selectable period ofdelay, in the course of which a temporary deficiency in supply could berectified, the relay 175 closes a contact 176. This in turn results inthe operation of the relay 170 and the plantis controlled in the sameway as when a breakthrough occurs.

in horizontal casting a fall in the level of the metal in the tundishcan be monitored for the purpose of generating a breakthrough signal.The breakthrough signal can be used to separate the tundish from themold by hydraulic actuating means. The liquid steel is then dischargedinto an emergency vessel located underneath the point of separation.

The above examples describe only several means for generating and usingfault signals. However, the invention is not intended to be limited inscope to these solutions, since more precise fault signals can begenerated by suitably combining the above described principles. Moreparticularly an arrangement may be provided comprising a rod source andseveral radiation detectors whereby any changes in the level of themetal can be indicated by making use of the varying signal strengthssupplied by the detectors and the time intervals at which consecutivesignals appear. Rapid and accurate information relating to the nature ofany fault can thus be obtained. For example, a breakthrough may thus beidentified in its initial stages from a relatively small change inlevel. By immediately intensifying the cooling effects such abreakthroughmay possibly even be healed in good time. if the change inlevel is appreciable, that is to say if the breakthrough has had time toprogress and is therefore incapable of being healed, then a secondbreakthrough signal may be arranged to increase the rate of withdrawalsufficiently to permit the billet to be withdrawn from the plant beforeit has frozen completely tight.

This invention may be variously modified and embodied within the scopeof the subjoined claims.

We claim:

1. A method of controlling a continuous casting plant during faultconditions which cause the level of the liquid metal surface in the moldto fall, comprising the steps of issuing an output fault signal whichindicates a fall of metal level unusual to normal casting conditionsbeyond a level limit which is given by normal casting conditions andcontrolling the plant with this fault signal according to requirementsto finish unusual casting conditions, and including the step ofgenerating a breakthrough signal when a breakthrough occurs, saidbreakthrough being generated when there is a fall of level of liquidmetal in the partly solidified casting.

2. A method of controlling a continuous casting plant during faultconditions which cause the level of the liquid metal surface in the moldto fall, comprising the steps of issuing an output fault signal whichindicates a fall of metal level unusual to normal casting conditionsbeyond a level limit which, is given by normal casting conditions andcontrolling the plant with this fault signal according to requirementsto finish unusual casting conditions, and including issuing a signalindicating an absolute minimum level of metal in a pouring vessel, whichlater signal is used for bridging fault signals at the end of thecasting operation.

3. A method according to claim 1, wherein a deficiency signal isgenerated when there is a fall in the level of the liquid metal poolunusual to normal casting conditions.

4. A method according to claim 1, wherein a teeming jet is formed whenthe liquid metal is poured into the mold, and wherein said deficiencysignal is derived according to the size of the teeming jet.

5. A method according to claim I, wherein said deficiency signal stopsthe supply of metal to the mold after a period of delay.

6. A method according to claim 1, wherein said continuous casting planthas withdrawing rollers and associated drive means for withdrawing thecasting, and wherein said deficiency signal stops the drive means of thewithdrawing rollers after a period of delay.

7. A method according to claim 1, wherein said continuous casting planthas a cutting device for cutting the casting, said v cutting devicebeing equipped with a synchronizing gear and a traversing gear, andwherein said deficiency signal stops the synchronizing gear of thecutter after a period of delay but permits a proceeding cuttingoperation to be completed by the traversing gear.

8. A method according to claim 1, wherein said deficiency signal changesover the control of the plant to manual control after a period of delay.

